Compact, two stage, zero flux electronically compensated current or voltage transducer employing dual magnetic cores having substantially dissimilar magnetic characteristics

ABSTRACT

A device for sensing electrical current or voltage in an electrical distribution system using an actively compensated current ratio transformer that includes a first magnetic core having a first permeability and a second magnetic core having a second permeability higher than the first permeability. A primary winding having P turns is coupled with the first and second magnetic cores, a measurement winding having M turns is coupled with the first and second magnetic cores so that current in the primary winding induces current in the measurement winding, and a sense winding having S turns is coupled with the second magnetic core. An amplifier coupled to the sense winding receives a voltage developed across the sense winding and produces a compensation current in response to the received voltage. The amplifier has an output coupled to the sense winding to feed the compensation current through the sense winding to reduce the voltage developed across the sense winding voltage to substantially zero. A burden resistor is coupled to the measurement winding and the sense winding for receiving the sum of the current induced in the measurement winding and the compensation current.

FIELD OF THE INVENTION

This invention relates to precision, alternating current and voltageratio transformation having primary utility in the accurate measurementof higher current or voltage signals applicable to the field of digitalpower measurement apparatus having fundamental application in 50-60Hertz AC power systems.

BACKGROUND

Traditional digital power meters typically employ conventional passiveinternal current transformers and resistive potential dividers in orderto reduce relatively large input currents and voltages by a defined andcalibrated ratio down to lower currents and voltages that are readilysampled and converted into a digital representation for further signalprocessing. Current transformers and resistive potential dividersadditionally provide much needed electrical isolation between theexternal current and voltage signals being measured. With potentialdividers, the isolation is afforded by providing a robust (transientoverload) and high divider impedance (typically >1 Meg ohm) between thevoltage source and the digital power meter input circuitry. Currenttransformation ratios of 1:1000 are common with (but not limited to)typical nominal primary current levels of 1, 5, or 20 Amps in the caseof transformer-connected power meters. Voltage transformation ratios of200:1 are common with (but not limited to) typical nominal voltageinputs ranging from 67 to 600 Vac. Accurate current and voltagetransformation, in both magnitude and phase, is required, particularlywhen AC power calculations are being made at low power factors.Amplitude error of less than +/−100 ppm, combined with phase shifterrors of less than +/−1 minute, are required by the newest generationof Class 0.1 digital power meters. Accuracies must be maintained overwidely varying signal amplitudes and environmental conditions. Accuracyat higher current and voltage signal frequencies well beyond fundamental60 Hz power signals are becoming common, particularly when harmonicrepresentation, power quality, and transient analysis is required.

Conventional current ratio transformers suffer from a fundamentalelectromagnetic limitation that directly impacts their effective use inmodern sophisticated digital power meters, particularly the new class ofpower quality meters requiring high accuracy (Class 0.1), wide dynamicrange, stability, and frequency response. This limitation is due to thefact that a portion of the primary input current being measured isrequired to magnetize the core. This magnetization current component iscomplex in magnitude and phase and directly impacts the ratio and phaseerror of the current ratio transformer output current. Core magetizationeffects may also impact accuracy by shifting the transformer flux swingoperating point. Larger, high permeability cores are typically used inorder to minimize the effects of core magnetization loss. Theseundesirable effects are only reduced and not eliminated through the useof such cores. Tape wound torroidal cores, made of ultra highpermeability magnetic alloys, such as Molypermalloy, Supermalloy, andAmorphous Glass, may be required to meet the 60 Hz accuracyspecifications, but issues of cost, size, and accuracy often limit theirinclusion in new high performance designs.

A conventional potential divider used for power meter AC input voltagedivision typically utilizes high valued resistors in order to safelydivide the input signal to low levels compatible with conventionalelectronic analog to digital conversion circuitry. The divider inputresistor values must also be of high value in order to limit powerdissipation under nominal and overload conditions while reducing leakagecurrents to safe levels. Unfortunately, the use of such high valueresistor divider chains can result in temperature, humidity, capacitive,and thermal noise induced stability issues. The use of high precisionmatched resistive dividers (e.g., metal foil) are generally required forhigh accuracy applications but come at a high cost factor.

The continuing trend of increased digital power meter performance,particularly in areas of accuracy and frequency response, requires a newand improved approach.

BRIEF SUMMARY

The present disclosure provides a device for sensing electrical currentor voltage in an electrical distribution system using an activelycompensated current ratio transformer that includes a first magneticcore having a first permeability and a second magnetic core having asecond permeability higher than the first permeability. A primarywinding having P turns is coupled with the first and second magneticcores and is connected to a source of current to be measured. Ameasurement winding having M turns is coupled with the first and secondmagnetic cores so that the current to be measured in the primary windinginduces current in the measurement winding, and a sense winding having Sturns is coupled with the second magnetic core. An amplifier coupled tothe sense winding receives a voltage developed across the sense windingand produces a compensation current in response to the received voltage.The amplifier has an output coupled to the sense winding to feed thecompensation current through the sense winding to reduce the voltagedeveloped across the sense winding voltage to substantially zero. Aburden resistor is coupled to the measurement winding and the sensewinding for receiving the sum of the current induced in the measurementwinding and the compensation current. The summing of the compensationcurrent with the current induced in the measurement winding preferablycompensates for magnetization losses in the first magnetic core, so thatthe voltage produced across the burden resistor is substantiallyproportional to the current to be measured in the primary windingmultiplied by the ratio P/M.

In one embodiment, the measurement winding has a greater number of turnsthan the sense winding, and an attenuation circuit attenuates thecompensation current, before the summing of the compensation currentwith the current induced in the measurement winding, to compensate forthe difference between the number of turns in the sense winding and thenumber of turns in the measurement winding.

In one implementation, the permeability of the second magnetic core isat least three times the permeability of the first magnetic core and issubstantially independent of temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an electrical schematic diagram of a current transformerembodying the invention.

FIG. 2 is an electrical schematic diagram of a voltage transformerembodying the invention.

FIG. 3 is a sectioned perspective view of an actively compensatedcurrent or voltage ratio transformer.

FIG. 4 is a block diagram of a digital power meter including the currentand voltage transformers of FIGS. 1 and 2.

DETAILED DESCRIPTION

Although the invention will be described in connection with certainpreferred embodiments, it will be understood that the invention is notlimited to those particular embodiments. On the contrary, the inventionis intended to cover all alternatives, modifications, and equivalentarrangements as may be included within the spirit and scope of theinvention as defined by the appended claims.

FIG. 1 shows an actively compensated current ratio transformer devicehaving a lower permeability first “main” core 10, and a higherpermeability second “sense” core 11 physically positioned in a stackedarrangement as shown in FIG. 3. The main core is made of a lower costand lower permeability material (such as a ferrite, e.g., Ferroxcube 3E6Ferrite), while the sense core 11 is made of a higher permeability metalamorphous core material (such as a base metal composition, e.g.,Vacuuschmelze Vitroperm). This combination maximizes accuracy andstability while maintaining a low overall component cost.

Referring to FIG. 1, a primary winding 12, having P turns, couplesmagnetically with both the lower permeability main core 10 and thehigher permeability sense core 11. For current transformer use, theprimary winding 12 is nominally, but not limited to, a single-turnconductor. The measurement current of interest is the primary windingcurrent Ip flowing in the primary winding 12.

A measurement winding 13, having M turns, also couples magnetically withboth the lower permeability first main core 10 and the second higherpermeability sense core 6. The turns ratio of the measurement winding 13to the primary winding 12 is nominally of high value in order to reducethe primary winding current Ip to an acceptable (galvanically isolated)lower level for measurement as a voltage developed across burdenresistor R4 as the result of a burden current Ib flowing through theburden resistor R4. Typical values of the primary winding current Ip,for current transformer use, range from 0 to 5A RMS 50/60 Hz intransformer-connected power metering applications.

A sense winding 14, having S turns, couples electromagnetically withonly the second higher permeability sense core 11.

In summary, the first lower permeability main core 10 iselectromagnetically coupled to two windings: the primary winding 12 andthe measurement winding 13. The second higher permeability sense core 11is electromagnetically coupled to three windings: the primary winding12, the measurement winding 13, and the sense winding 14.

FIG. 2 shows an actively compensated voltage ratio device havingphysical and electrical topology that is substantially the same as thatof the current ratio transformer of FIG. 1, with the addition of aseries voltage dropping resistor R3. An increase in the number (P=1000)of primary winding turns is combined with an increase in the impedanceof the burden resistor R4 (e.g., in 100 ohms). An external AC voltagesource Vs is applied to the primary winding 2. The number of turns forall the windings, and the values of all the components, may be selectedfor specific uses (to accommodate specific input/output voltage andcurrent levels).

The output of the sense winding 14 is connected to the high impedanceinverting and non-inverting inputs of a high gain voltage operationalamplifier 20, the voltage output of which drives a compensation currentIc through the sense winding 14. The sense winding compensation currentIc is reduced in level through a divider formed by resistors R1 and R2,and applied as a measurement winding current Im to the measurementwinding circuit. A pair of parallel diodes D1 and D2 connected acrossthe inputs of the amplifier 20 protect the input of the operationalamplifier 20 from possible transient primary winding over-range signalconditions. A capacitor C1 connected across the inputs of the amplifier20 provides compensation circuit stability.

The operational amplifier 20 is provided with power from a +5 Vdcvoltage supply 21 (e.g., +5 Vdc). A virtual ground reference DC voltagesupply 22 (e.g., +2.5 Vdc) provides a reference source effectivelybiasing the static DC operating point of the output of the operationalamplifier 20 to a voltage level that centers the output swing within therange of the primary supply 21. It will be appreciated that other supplyand reference source supply configurations are possible withoutaffecting the underlying circuit operation. The example shown here isbased on having a single unipolar operational amplifier supply 21. Dualsupply and ground referenced offset voltage sources can easily beaccommodated depending on the specific supply voltage levelavailability.

For current transformer operation, the AC current being measured(FIG. 1) is applied to the input of the primary winding 12 having Pturns and flows as the primary winding current Ip.

Through transformer action, a secondary current Is develops in themeasurement winding 5 having M turns and is represented by the followingequation:

$\overset{harpoonup}{Is} = {\frac{Ip}{\frac{M}{P}} - \overset{harpoonup}{Im}}$

The measurement winding current Im represents the current required tomagnetize the lower permeability main core 10 and arises due totransformer and main core losses. The current Im is a complex vectorquantity of varying magnitude and phase, having non-linear sensitivityto the characteristics of the core material, operating temperature, andcore flux level. Without active compensation, the resulting secondarycurrent and the voltage across the burden resistor R4 have unacceptableratio and phase errors, as referenced to the primary current Ip. Theseerrors are unacceptable for high accuracy power metering applications.

It will be noted that the standard dot convention is used in FIGS. 1 and2 to indicate the direction of each winding relative to the otherwindings in the transformer. Voltages at the dot end of each winding arein phase, while current flowing into the dot end of a primary coil willresult in current flowing out of the dot end of a secondary coil.

The operational amplifier 20 is arranged with its inverting andnon-inverting inputs connected directly across the sense winding 14output which is electromagnetically linked to only the higherpermeability sense core 11. The operational amplifier 20 operates toeffectively reduce to zero any voltage appearing across the sensewinding 14 through a feedback connection made between the output and theinverting input of the amplifier 20, and connected to one end of thesense winding 14. The sense winding 14 compensation current Ic developsto force the sense winding voltage output to zero. By Faraday's law ofinduction, having zero output voltage from the sense winding 14 impliesa zero time-varying flux condition in the high permeability sense core11. The high permeability sense core 11 is therefore operating, throughactive compensation, at close to zero flux, and thus experiences verylow core losses. Non-ideal operational amplifier characteristics (finitegain, noise, offsets), winding copper losses, and flux leakage pathsprevent complete reduction of sense core 11 operating flux. The use of ahigh permeability (and stable) material for the sense core 11 keepsresidual losses (and errors) at very low levels. The very low fluxdensity in the sense core 11 allows for a small sense core magneticcross section, thereby maintaining low overall costs when using moreexpensive higher permeability materials.

The sense winding compensation current Ic effectively removes theprimary winding-to-measurement winding ampere-turn imbalance since boththese windings also link the higher permeability core 11. Theampere-turn imbalance is due to the measurement winding current Imrequired to magnetize the lower permeability core. Im is numericallyequal to:

$\overset{harpoonup}{Im} = \frac{\overset{harpoonup}{Ic}}{\frac{M}{S}}$

S is the number of sense winding turns and M is the number ofmeasurement winding turns. It is advantageous to have a reduced numberof sense winding 14 turns in order to reduce both manufacturing costsand winding resistance, especially when higher primary to secondaryratios exist, i.e. when M measurement winding turns are high. The effectof the compensation current Ic through the resistive voltage drop acrossthe sense winding 14 is minimized by using a lower number of sensewinding turns. It should be noted that the finite gain and drivecapability of the operational amplifier 20 establishes a lower limit tothe number S of sense winding turns.

The compensation current Ic is reduced by a current divider formed byresistors R1 and R2 before injection into the measurement windingcircuit as Im. The following equality applies:

$\frac{S}{M} = \frac{R\; 2}{{R\; 1} + {R\; 2}}$

Active compensation is completed through the injection of electronicallyderived current Im into the measurement winding circuit, effectivelyreplacing the magnetization current component lost in magnetizing thelower permeability main core 10. Under conditions of activecompensation, the ampere-turns of the primary winding 14 is in precisebalance with the ampere-turns of the secondary measurement winding 13and therefore the resulting burden current Ib is related to the primarycurrent Ip by a constant factor of M/P (measurement winding to primarywinding turns ratio). Ratio and phase errors are therefore essentiallyremoved from the burden current Ib and the resultant output voltagedeveloped across the burden resistor R4. It will be appreciated that thecorrect current or voltage transformer winding polarity relationship ismandatory for proper active compensation to occur.

For potential transformer operation (FIG. 2), the primary windingcurrent Ip equals the input voltage Vs divided by the sum of an inputresistor R3 and the reflected burden impedance R4/(M/P)². With P=M, theturns ratio is unity and therefore the reflected impedance (varies asthe square of the turns ratio) is simply equal to the value of R4. Thefollowing equation shows the relationship of Ip to input voltage Vs:

${Ip} = \frac{Vs}{{R\; 3} + \frac{R\; 4}{( \frac{M}{P} )^{2}}}$

FIG. 3 shows the physical construction of the actively compensatedcurrent or voltage ratio transformer having a stacked toroidalarrangement of the cores 10 and 11 to help minimize leakage flux andimprove the self-shielding characteristics of the compensated current orvoltage ratio transformer combined with a measurement winding 13 and asense winding 14. (Other arrangements are possible, including atoroid-within-a-toroid arrangement but are generally more expensive andcomplex than needed for a power metering application.) The measurementwinding 13 is wound over both cores 10 and 11, while the sense winding14 is wound only over the sense core 11. The primary winding 12 is shownas a single turn in the current transformer embodiment and passesthrough the central tunnel 30 of the stacked toroidal core combination.

The voltage transformer embodiment typically requires many primary turns(not shown in FIG. 3) which are wound over an insulating layerpositioned directly on top of an electrostatic and magnetic stampedmetal shield 31 that completely covers the current ratio transformerincluding the internal surface of the axial tunnel 30. A small air gap32 prevents the shield from forming a shorted secondary turn. The shield31 operates to prevent stray electrostatic and higher frequency magneticfields from coupling to the windings and/or the main and sense cores.The measurement winding 13 and the sense winding 14 are brought out astwo conductor pairs 33 through a small opening 34 in the outer shield31. A shield ground connection wire 35 is provided with one endphysically soldered to the shield 31. The illustrative embodimentemploys a significant number of machine wound turns requiring the use offine copper wire (e.g., 34 AWG) in order to construct a commerciallyviable compact and cost effective transducer.

FIG. 4 illustrates a typical application of the actively compensatedcurrent and voltage ratio transformers as employed in a digital powermeter. For schematic simplicity, the block diagram of FIG. 4 illustratesthe key functional sections with a single phase voltage and current pair(phase A) shown. It will be appreciated that for polyphase applications,a simple duplication of the analog circuitry is all that is required(from a hardware standpoint) for additional voltage and current phasepairs.

The primary input current Ip is applied to an actively compensatedcurrent ratio transformer 40 as previously described and shown inFIG. 1. The compensated burden resistor voltage output is applied to aseries of fixed gain amplifiers 41 ranging from a high gain CREEP stageto a lower gain OVER_RANGE stage. The outputs of these amplifiers 41 areapplied to the multiplexed inputs of a current A/D converter 42. Thespecific selected input is controlled by a digital signal processor 43operating to select the required range based on current signal levels.This auto-ranging capability utilizes the wide dynamic range offered bythe actively compensated current ratio transformer topology.

The corresponding voltage phase is applied to an actively compensatedvoltage ratio transformer 44 as previously described and shown in FIG.2. The output drive dual-range-gain amplifiers 45 in a similar fashionto the corresponding current channel. The outputs of the amplifiers 45are applied to the multiplexed inputs of a voltage A/D converter 46.Both the current and voltage A/D converters 42 and 46 are simultaneouslysampled with the acquired digital waveform representation processed inreal time by the digital signal processor 43 and a main CPU 47. Powermeasurement quantities, such as real power (watts), reactive power(VARS), energy (watt-hrs), volts (RMS), current (RMS) and power factor,are provided to the user through a display I/O 48 and a digital COMM 49.A large memory bank 50 is used for storage of variables, waveforms andprograms.

While particular embodiments and applications of the present inventionhave been illustrated and described, it is to be understood that theinvention is not limited to the precise construction and compositionsdisclosed herein and that various modifications, changes, and variationsmay be apparent from the foregoing descriptions without departing fromthe spirit and scope of the invention as defined in the appended claims.

1. A device for sensing electrical current in an electrical distributionsystem, said device comprising: an actively compensated current ratiotransformer comprising: a first magnetic core having a firstpermeability and a second magnetic core having a second permeabilityhigher than said first permeability, a primary winding comprising Pturns coupled with said first and second magnetic cores and connected toa source of current to be measured, a measurement winding comprising Mturns coupled with said first and second magnetic cores so that saidcurrent to be measured in said primary winding induces current in saidmeasurement winding, a sense winding comprising S turns coupled withsaid second magnetic core, an amplifier coupled to said sense windingfor receiving a voltage developed across said sense winding andproducing a compensation current in response to said received voltage,said amplifier having an output coupled to said sense winding to feedsaid compensation current through said sense winding to reduce saidvoltage developed across said sense winding voltage to substantiallyzero, and a burden resistor coupled to said measurement winding and saidsense winding for receiving the sum of said current induced in saidmeasurement winding and said compensation current.
 2. The device ofclaim 1 in which the summing of said compensation current with saidcurrent induced in said measurement winding compensates formagnetization losses in said first magnetic core, so that the voltageproduced across said burden resistor is substantially proportional tosaid current to be measured in said primary winding multiplied by theratio P/M.
 3. The device of claim 2 in which the ratio P/M is asubstantially in-phase and real-valued proportionality constant.
 4. Thedevice of claim 1 in which the permeability of said second magnetic coreis substantially independent of temperature.
 5. The device of claim 1 inwhich said measurement winding has a greater number of turns than saidsense winding, and which includes an attenuation circuit receiving saidcompensation current and attenuating said compensation current, beforethe summing of said compensation current with said current induced insaid measurement winding, to compensate for the difference between thenumber of turns in said sense winding and the number of turns in saidmeasurement winding.
 6. The device of claim 5 in which S issubstantially smaller than M.
 7. The device of claim 1 in which saidcurrent induced in said measurement winding magnetization current issubstantially equal to said compensation current multiplied by S/M. 8.The device of claim 5 in which said amplifier comprises an operationalamplifier having non-inverting and inverting inputs coupled to oppositeends of said sense winding, and an output, said inverting input and saidoutput being coupled to the non-dotted end of said sense winding, and inwhich said attenuation circuit comprises a first resistor connectedbetween said non-inverting input and the dotted end of said measurementwinding, and a second resistor connected between said non-invertinginput and a reference point.
 9. The device of claim 8 in which saidfirst resistor has a value R1 and said second resistor has a value R2,and the ratio S/M is substantially equal to the ratio of R1/(R1+R2). 10.The device of claim 1 which includes a diode clamp across said sensewinding.
 11. The device of claim 1 in which M is substantially equal to1000 and S is substantially equal to
 258. 12. The device of claim 1 inwhich said first magnetic cores comprise ferrite material, and saidsecond magnetic core comprises a base metal composition.
 13. The deviceof claim 1 which includes an electrostatic and magnetic metal shieldsurrounding said actively compensated current ratio transformer.
 14. Thedevice of claim 13 which includes a ground connection wire bonded tosaid electrostatic and magnetic metal shield.
 15. A device for sensingelectrical voltage in an electrical distribution system, said devicecomprising: an actively compensated voltage ratio transformercomprising: a first magnetic core having a first permeability and asecond magnetic core having a second permeability higher than said firstpermeability, a primary winding comprising P turns coupled with saidfirst and second magnetic cores and connected to a source of voltage tobe measured, a measurement winding comprising M turns coupled with saidfirst and second magnetic cores so that currents produced in saidprimary winding by said connection to said voltage source inducescurrent in said measurement winding, a sense winding comprising S turnscoupled with said second magnetic core, an amplifier coupled to saidsense winding for receiving a voltage developed across said sensewinding and producing a compensation current in response to saidreceived voltage, said amplifier having an output coupled to said sensewinding to feed said compensation current through said sense winding toreduce said voltage developed across said sense winding voltage tosubstantially zero, and a burden resistor coupled to said measurementwinding and said sense winding for receiving the sum of said currentinduced in said measurement winding and said compensation current. 16.The device of claim 15 in which the summing of said compensation currentwith said current induced in said measurement winding compensates formagnetization losses in said first magnetic core, so that the voltageproduced across said burden resistor is substantially proportional tosaid current to be measured in said primary winding multiplied by theratio P/M.
 17. The device of claim 16 in which the ratio P/M is asubstantially in-phase and real-valued proportionality constant.
 18. Thedevice of claim 15 in which the permeability of said second magneticcore is substantially independent of temperature.
 19. The device ofclaim 15 in which said measurement winding has a greater number of turnsthan said sense winding, and which includes an attenuation circuitreceiving said compensation current and attenuating said compensationcurrent, before the summing of said compensation current with saidcurrent induced in said measurement winding, to compensate for thedifference between the number of turns in said sense winding and thenumber of turns in said measurement winding.
 20. The device of claim 19in which S is substantially smaller than M.
 21. The device of claim 15in which said current induced in said measurement winding magnetizationcurrent is substantially equal to said compensation current multipliedby S/M.
 22. The device of claim 19 in which said amplifier comprises anoperational amplifier having non-inverting and inverting inputs coupledto opposite ends of said sense winding, and an output, said invertinginput and said output being coupled to the non-dotted end of said sensewinding, and in which said attenuation circuit comprises a firstresistor connected between said non-inverting input and the dotted endof said measurement winding, and a second resistor connected betweensaid non-inverting input and a reference point.
 23. The device of claim22 in which said first resistor has a value R1 and said second resistorhas a value R2, and the ratio S/M is substantially equal to the ratio ofR1/(R1+R2).
 24. The device of claim 15 which includes a diode clampacross said sense winding.
 25. The device of claim 15 in which M issubstantially equal to 1000 and S is substantially equal to
 258. 26. Thedevice of claim 15 in which said first magnetic cores comprise ferritematerial, and said second magnetic core comprises a base metalcomposition.
 27. The device of claim 15 which includes an electrostaticand magnetic metal shield surrounding said actively compensated currentratio transformer.
 28. The device of claim 27 which includes a groundconnection wire bonded to said electrostatic and magnetic metal shield.